The industry of lithium ion cell develops quickly since SONY corporation of Japan invented and commercialized a secondary lithium ion cell. Up to 2000, the manufactures of lithium ion battery around the world compete allsidedly for improving the competitive power of their products mainly around the key issue, the capacity of lithium ion battery. At present, the improvement of capacity of commercialized secondary lithium ion battery generally depends on the increase of loading quantities of active substances (positive electrode materials and negative electrode materials). However, the limitation of the volume of lithium ion battery greatly restricts the increase of the battery capacity. For notably raising the capacity, the researches for the development of active substances (positive electrode materials and negative electrode materials) having higher specific energy are been conducting around the world, but so far, there is no notable breakthrough in this aspect for various technical difficulties.
In fact, the positive electrode materials and negative electrode materials used in the current secondary lithium ion battery have relatively higher theoretical capacity, and the problem merely lies in the lower actual utilization rate of said capacity. For example, lithium cobalt oxides as a positive electrode material of secondary lithium ion cell has a theoretical capacity of 248 mAh/g, while the actually used capacity of it is merely about 140 mAh/g, i.e., about half of said theoretic capacity is not utilized. This is mainly caused by the limitation of charge cut-off voltage commonly used in the art. At present, the charge cut-off voltage of single secondary lithium ion cell is limited to no more than 4.2 V, and this is well accepted as a technical requirement in the industry of manufacture of secondary lithium ion battery. Further, all lithium ion batteries in the markets around the world are manufactured under this technical requirement For example, the charge cut-off voltage is limited to below 4.2 V and the overcharge release voltage of its protection circuit is controlled below 4.15 V during the formation of single lithium ion cell. The reasons that the charge cut-off voltage being limited to below 4.2 V lie in the following opinions in the prior research results and documents: although the capacity and average operating voltage are improved by increasing the charge cut-off voltage, the positive electrode materials and the negative electrode materials will undergo structure change, the electrolyte may decompose, and the recycle property of the cell will be adversely affected when the charge cut-off voltage is greater than 4.2 V.
For instance, as to lithium cobalt oxides that is used as positive electrode material in the most commercial lithium ion batteries, the charge cut-off voltage is limited to below 4.2 V and the actual capacity is 120-140 mAh/g, i.e., about 50% of the theoretical capacity, although many documents indicate that the charge cut-off voltage can be over 4.2 V for a test cell using metallic lithium as counter electrode. In fact, according to MIZUSHIMAK et al., “A new cathode material for batteries of high energy density”, Mater. Res. Bull., 1980, 15:783, the quantity of dedoped lithium ion increases with the increase of charge voltage, and the electrochemical capacity of lithium cobalt oxides increases accordingly. However, the study deems that the reversible charge-discharge voltage is about 4.3 V when metallic lithium is used as counter electrode, and when said voltage is higher than 4.3 V, the structure of lithium cobalt oxides changes and the lattice parameter C decreases from 4.4 nm to 4.0 nm, and thus the recycle life of cell is affected.
G. PISTOIA et al., J. Power Source, 56(1995), 37-43, deems that the structure the lithium cobalt oxides changes with the charge voltage, and the coexistence of monoclinic phase and hexagonal phase will appear when the charge voltage is over a certain value, which will spoil the recycle property of cell. The results of experiments showed that as to a test button cell having metallic lithium as negative electrode, the capacity of lithium cobalt oxides reaches 159 mAh/g when the charge cut-off voltage is 4.35 V, but it drops to 135 mAh/g after several cycles; and the capacity attenuates quickly when the charge cut-off voltage is 4.25 V. This document takes the opinion that lithium cobalt oxides maintains excellent recycle property and a capacity about 130 mAh/g only when the charge cut-off voltage is 4.15 V, and the corresponding voltages of monoclinic phase and hexagonal phase separately is 4.05 V and 4.17 V, i.e., both of them are below 4.2 V.
In addition, Lei Yongquan, “Materials for New Energy” (in Chinese), 2000, p136, discloses that the decomposition voltage of electrolyte solution using LiPF6 as electrolyte and EC/DMC as mixture solvent is 4.2 V, and thus deems that the electrolyte solution will be decomposed and the recycle life will be affected when the charge cut-off voltage is above 4.2 V.
In 1990, Sony Corporation issued the lithium ion cell using coke as negative electrode, which has a charge cut-off voltage of not more than 4.20 V, and it is accepted as a common technical requirement of lithium ion cells thereafter.
The prior art deems:                1. The increase of charge cut-off voltage will change the structure of positive electrode material, which mainly exhibits at the following two aspects: one aspect is that the phase change, i.e., the coexistence of monodinic phase and hexagonal phase and the conversion between them may seriously affect the recycle life of lithium ion cell; and another aspect is that the change of lattice parameter may narrow the channel for passing lithium ions, squeeze the space occupied by lithium ions, jam the channel of lithium ions, and decrease the recycle property of lithium ion cell.        2. The elevated charge cut-off voltage may decompose the electrolyte solution, and the loss of electrolyte solution renders the transportation of lithium ion more difficult, and thus the recycle life of cell is seriously affected.        
Therefore, it can be seen that the limitation of charge voltage restricts the actual utilization of active electrode materials. Under this condition, even if new positive electrode material and negative electrode material having higher specific energy are developed, the lithium ion cell cannot exhibit the best performance. Hence, it is urgently needed to provide a method that can improve the efficacy of active substances of lithium ion cell, consequently increase the capacity and average operating voltage, and maintain the better cell performance simultaneously.